This invention relates to apparatus for mixing liquids or liquid with particles to form slurries and the like. The apparatus of the present invention is suitable for mixing one liquid with another or mixing liquid with particles to form both homogeneous suspensions as well as mixtures in which not all of the particles are fully suspended. The invention is intended for applications where entrainment of gas from the liquid surface during mixing is undesirable and to be avoided.
Apparatus for mixing of this type has a number of applications in a wide variety of industrial processes. One such application is agitated precipitators used in the process of precipitating crystals from a supersaturated liquor. Precipitators of this type are used in a number of industrial processes. The invention will hereinafter be specifically described with reference to this application but it will be readily appreciated that the scope of the invention is not limited to this particular application.
One well known agitating precipitator is the Gibbsite precipitator used in the Bayer process to produce alumina hydrate from bauxite. Existing Gibbsite precipitators comprise a large vessel with a centrally disposed draft tube. An impeller is rotationally driven in the draft tube to provide a vertical circulation in the precipitator. In some cases baffles are provided around the sides of the vessel to prevent swirling or rotational flow in the slurry which otherwise impairs the desired vertical circulation. Existing Gibbsite precipitators use a large amount of input power to achieve the required circulation. Additionally, one of the objects of the precipitation process is to produce large crystal size in the precipitate. Because the existing Gibbsite precipitators involve a fairly energetic process as the slurry is drawn through the draft tube, there is a tendency to break crystal structures. This limits the size of the crystals that can be produced using these precipitators. Another difficulty with Gibbsite precipitators is the scaling that occurs on the precipitator walls due to the low flow velocities. In particular,a substantial deposition of material occurs in the bottom of the vessels and in the areas of stagnant flow. As a consequence, the vessels need to be periodically cleaned. Not only is cleaning an additional expense, but also provides a significant disruption to production and can reduce the life of the vessels.
Similar difficulties, in particular, the large power requirements exist in other apparatus for mixing liquids and liquids with particles in various industrial situations.
It is an object of this invention to provide a method and apparatus for mixing liquids and liquid with particles without entrainment of gas from the liquid surface which will overcome, or at least ameliorate, one or more of the forgoing disadvantages.
In one aspect this invention provides an apparatus for mixing liquids or liquid with particles without entrainment of gas from the liquid surface, said apparatus including a vessel to contain the liquid(s) having an upper end, a lower end and a containing wall extending between the upper and lower ends, a mechanical rotating device disposed adjacent said upper end and submerged in said liquid(s) to induce a rotational flow in the liquid directed radially outward from a central region of the vessel towards said containing wall to establish a swirling flow through the vessel characterised by an outer annular region of moderate rotational flow adjacent the containing wall moving from the upper end toward the lower end, an inward flow adjacent the lower end of the vessel, and an inner core region of rapid rotational flow about the central region of the vessel moving from the lower end toward the upper end and extending from substantially adjacent the lower end of the vessel to the mechanical rotating device.
In a further aspect this invention provides a method of mixing liquids or liquid with particles without entrainment of gas from the liquid surface, said method including the steps of placing the liquid(s) in a vessel having an upper end and a lower end and a containing wall extending between the upper and lower ends, inducing with a mechanical rotating device submerged in the liquid(s) in the part of the vessel adjacent the upper end a rotational flow in the liquid(s) directed radially outward from a central region of the vessel toward the containing wall to establish a swirling flow through the vessel characterised by an outer annular region of moderate rotational flow adjacent the containing wall moving from the upper end toward the lower end, an inward flow adjacent the lower end of the vessel, and an inner core region of rapid rotational flow about the central region of the vessel moving from the lower end toward the upper end and extending substantially from adjacent the lower end of the vessel to the mechanical rotating device.
In the swirling flow induced by the mechanical rotating device according to this invention the rotational flow is preferably about zero at the centre of the inner annular region and greatest toward the outer edge of that region.
Preferably, the mechanical rotating device inducing the rotational flow includes a paddle or impeller. The paddle or impeller preferably rotates about a central axis. The paddle or impeller preferably only operates in the central region of the vessel. Preferably the blades of the paddle or impeller extend from a central hub or are otherwise outwardly offset from the axis of rotation.
The vessel preferably has a circular cross-section. In one form of the invention a conical base section joins the containing wall toward the lower end of the vessel. In another form the base is flat. Preferably, the rotational speed of the paddle or impeller used to induce the flow is selected to achieve the desired flow velocities. Preferably, the liquid velocity adjacent the containing wall (outside the boundary layer) is between about 0.3 m/s and 1 m/s. Most preferably this velocity is greater than 0.5 m/s. In aluminia precipitators this has been found to ensure there is no scale build up on the precipitator walls. Maximum liquid tangential velocity in this inner core is preferably about 3 times the velocity adjacent the containing wall.
The present invention has particular application to vessels that have a height equal to or greater than the diameter of the vessel. The present invention has been found to provide satisfactory mixing in vessels having heights equal to and up to four times the diameter. Many prior art mixing devices are unable to provide satisfactory mixing in these configurations.
Preferably, the apparatus includes a device to provide a through flow of liquid through the vessel. Preferably, the device enhances the rotation of the liquid in the vessel.
In one specific application the invention provides a precipitator including a vessel having a smoothly continuous vertical wall at least in a horizontal direction to contain a slurry, a mechanical rotating device disposed in the upper part of said vessel and submerged in the slurry to induce a rotational flow in the slurry directed radially outward from the centre of the vessel to establish a swirling flow of the slurry through the vessel characterised by an outer annular region of downwardly moving moderate rotational flow adjacent the vertical wall, an inward flow across the bottom of the vessel, and an inner core region of upwardly moving rapid rotational flow about the centre of the vessel extending substantially from the bottom of the vessel to the mechanical rotating device.
Also in a specific application the invention provides a method of precipitating from a slurry including the steps of placing the slurry in a vessel having a smoothly continuous vertical wall at least in a horizontal direction, inducing in the upper part of the vessel with a mechanical rotating device submerged in the slurry a rotational flow in the slurry directed radially outwardly from the centre of the vessel to establish a swirling flow through the vessel characterised by an outer annular region of downwardly moving moderate rotational flow adjacent the vertical wall, an inward flow across the bottom of the vessel, and an inner core region of upwardly moving rapid rotational flow about the centre of the vessel extending substantially from the bottom of the vessel to the mechanical rotating device.
According to another improvement possible with this invention it is possible to operate the mixing apparatus on a non-continuous basis. This can be achieved by operating the mechanical rotating device used to induce the flow for example until an equilibrium is reached. and then allowing the momentum of the liquid to continue mixing until rotation decays to a predetermined level or for a set period at which time the paddle or a propeller is again operated. This process can allow a considerable reduction in power requirements particularly if it is possible to minimise the amount of time that power is required to be delivered during periods of peak cost of electrical power.
Preferably, the input power to the precipitator is less than 20 Watts/cubic meter. Power inputs as low as 7 or 8 Watts/cubic meter can maintain the suspension and mixing performance.
A further advantage of the invention is that solid material which would settle at the bottom of the vessel following a shutdown is more easily resuspended.
It has also been found that when the apparatus of the present invention is used as a precipitator an advantage can be obtained in terms of yield by the increased natural cooling due to absence of scale and increased fluid velocity over the walls and floor. In addition, cooling the walls of the vessel with water during operation can further enhance this effect.
A significant difference between the method and apparatus of this invention and prior art mixers resides in the intentional creation of the swirling or rotational flow. In prior art devices such flow is considered undesirable and baffles have been used to prevent it being established. Additionally, in accordance with the present invention the mechanical rotating means is submerged in the liquid. This prevents unwanted entrainment of gas from the liquid surface. The submerged mechanical rotating means also prevents waves or xe2x80x9csloshingxe2x80x9d on the surface of the liquid.
The invention will now be described, by way of example only, with reference to the accompanying drawings.